1. Field of the Invention:
The present invention relates to a system for driving a turbocharger installed in an internal combustion engine mounted on a motor vehicle such as an automobile, and more particularly to a drive system for a turbocharger combined with a rotary electric machine which can operate as an electric motor depending on operating conditions of the internal combustion engine.
2. Description of the Related Art:
Japanese Laid-Open Patent Publication No. 62-48931 discloses a turbocharger installed in an internal combustion engine on a motor vehicle and having a turbine shaft on which there is mounted a rotary electric machine that can operate alternatively as either an electric motor or a generator. The rotary electric machine is controlled to operate as the motor or the generator based on a signal indicative of the load on the engine and a signal indicative of the rotational speed of the engine. According to this known device when the engine is subjected to a partial load, the rotary electric machine is operated as the generator, and generated electric power is used to charge a battery. When the engine rotates at a low speed while under a high load, stored electric power is supplied from the battery to the rotary electric machine to operate the latter as the motor. The rotation of the turbocharger caused by the energy of exhaust gases from the engine is now assisted to increase the boost pressure for enabling the engine to produce a higher output power at the low engine speed under the high engine load.
A large amount of electric power is required to drive the rotary electric machine. When the engine operates at a low speed, especially if the engine is a gasoline engine, since the energy of emitted exhaust gases is small, a large amount of electric power must be supplied from the battery to drive the turbocharger sufficiently to assist it in supercharging the engine. Accordingly, the voltage of the battery tends to drop largely to the point where various electric parts on the motor vehicle may not be energized, and the battery may have a short service life.